CN113167182B

CN113167182B - Method and apparatus for increasing exhaust temperature in a diesel engine

Info

Publication number: CN113167182B
Application number: CN201980080226.8A
Authority: CN
Inventors: 马茨·海德曼
Original assignee: Hedman Ericsson Patent AB
Current assignee: Hedman Ericsson Patent AB
Priority date: 2018-12-14
Filing date: 2019-12-13
Publication date: 2023-06-20
Anticipated expiration: 2039-12-13
Also published as: EP3894679C0; US20210355888A1; MX2021007060A; SE1851587A1; KR20210099640A; EP3894679B1; EP3894679A4; SE543587C2; US11242810B2; ES2958159T3; BR112021009340A2; EP3894679A1; CN113167182A; WO2020122807A1; JP2022513220A

Abstract

The present invention relates to a method for providing increased exhaust gas temperature and reduced emissions at part load of a diesel engine, wherein the engine comprises a cylinder having a reciprocating piston, a variable compression Volume (VCR), at least one exhaust valve and at least one intake valve. The intake valve has a variable valve timing (WT). Based on prevailing engine power demands, the engine control system determines when to open and close the intake valve, and the size of the compression volume, to achieve a sufficiently elevated exhaust gas temperature so that proper exhaust gas purification can be achieved. The method is characterized in that the cylinder pressure during the expansion stroke is managed by the engine control system by means of VCR and VVT functions such that at an engine load of 25% or below the maximum engine load, said pressure reaches atmospheric pressure or a level below atmospheric pressure at or before bottom dead centre, whereby the inlet valve is opened to allow air to mix with combustion gases. The invention also relates to a corresponding device and a diesel engine comprising said device.

Description

Method and apparatus for increasing exhaust temperature in a diesel engine

Technical Field

The present invention relates to a method of providing increased exhaust gas temperature and/or reduced emissions during partial engine load of a diesel engine and to an apparatus for performing the method.

Background

It is well known that emission control of diesel engine vehicles performs poorly at lower speeds, such as in urban traffic or in frequent start and stop. This is particularly effective during start-up and initial driving using a cold machine.

In the publication on emission control in swedish traffic agency chapter 11, 10 of 2012, the following describes the background, the state of the art and the problems of today's diesel engines in a sufficient and complete manner.

We cite:

"11 emission control

Exhaust emission and emission control

During combustion in a diesel engine, different types of exhaust emissions are formed. Some of which are under the control of legal regulations. Since the introduction of law, the requirements have become more and more stringent. The emissions controlled are hydrocarbon emissions (HC), carbon monoxide (CO), nitrogen oxides (NOx) and Particulates (PM). Carbon dioxide is also emitted during combustion in diesel engines, but this gas is a combustion product whose production depends on the number of carbon atoms in the fuel. Carbon dioxide is a gas that causes the greenhouse effect and, if it is generated during the combustion of fossil fuels, is subject to tax, the so-called carbon dioxide tax.

The basic goal of all emission-related engine developments has long been to reduce the basic emissions of the engine, i.e. to reduce emissions formed in the engine combustion system. It has been very successful for many years by developing advanced combustion technologies, injection systems and sequential injections with extremely high pressures, and advanced gas exchange by developing turbocharger systems. This is increasingly integrated with the electronic control systems of the engines and their components.

However, during the last decade, emissions requirements have become more stringent, which has led to the development of different types of exhaust aftertreatment systems. These systems further reduce emissions behind the engine.

One of the most difficult problems is reducing particulate and NOx emissions. NOx is the result of oxidation between oxygen and nitrogen in air and increases rapidly with combustion temperature. High temperature combustion may reduce the formation of HC, CO, and particulates and help reduce fuel consumption, but may increase NOx emissions.

Method for reducing NOx

The need to reduce NOx emissions from heavy duty diesel engines has become more stringent over the years. From the first emission standard (European I1992) to today (European VI 2013), the limit has been reduced by 95%.

It has not been possible to achieve this reduction by improved combustion techniques, but rather it is necessary to develop separate technical solutions.

Exhaust Gas Recirculation (EGR)

The method is based on a portion of the exhaust gas being cooled and recirculated to the intake of the engine and then further into the cylinders. These exhaust gases reduce the formation of NOx, because on the one hand the concentration of oxygen is reduced and on the other hand the gases in the cylinder are cooled by the exhaust gases. This results in a reduction in the combustion temperature of the flame itself, thereby reducing the formation of NOx.

This method is effective, but the amount of recirculated exhaust gas must be controlled in accordance with the engine speed and load. The disadvantage is that the formation of particles often increases and this method results in a higher load on the particle filter. This technique increases the fuel consumption of the engine to some extent.

SCR-selective catalytic reduction

SCR refers to mounting an SCR catalyst behind an engine. A tank containing urea solution is mounted on the vehicle. The solution is sprayed from the tank into the exhaust pipe in front of the catalyst. After proper mixing, nitrogen oxides in the exhaust gas are converted to nitrogen and water. The urea injection is electronically controlled and varies according to engine load and speed. Catalytic reduction of NOx means that the catalyst and the reducing agent added before the catalyst are converted into nitrogen (N2) and oxygen (O2) (water: H2O). The most common reductant is ammonia (NH 3), typically in the form of urea. Urea

Stored on board the vehicle and converted to ammonia in combination with reduction.

A requirement for the operation of SCR systems is that the exhaust gas temperature must be sufficiently high. If the exhaust temperature falls below about 200 deg.c, the SCR system will no longer function and NOx reduction will be stagnant. The reduction is about 90-95% at about 300 ℃. Another requirement is that there must be sufficient oxygen in the exhaust gas. There are also systems with air-assisted urea injection which on the one hand atomize the urea solution, so that the spray is more atomized and used more effectively. The method also ensures that the oxygen content in the exhaust gas is at a suitable level.

In addition to reducing NOx, SCR catalysts can also reduce particulates and HC on diesel engines. HC emissions may be reduced by up to 80% and particulate matter may be reduced by up to 20-30%.

Operating an SCR system involves several technical challenges. Several examples are: urea and its dosage complex treatments, catalysts require high temperatures to operate effectively, control excess ammonia under transient conditions, and catalyst size. Ammonia in the surrounding air causes secondary particles and so "ammonia slip catalyst" should also be used. SCR releases more fine particles and is therefore typically used in conjunction with particulate filters.

Reduction of CO and HC emissions

The CO emissions of diesel engines have caused relatively minor problems, as diesel engines involve combustion by excess air. However, during the start-up and heating phases, the HC emissions of the engine may be high. During normal operation, these emissions are typically very low.

However, the use of an oxidation catalyst readily reduces emissions of CO and HC. Such catalysts require an excess of oxygen in the exhaust gas, which is what diesel engines have. With the aid of this oxygen, CO, HC and HC derivatives are oxidized to CO2 and water vapor. A disadvantage is that in order for the catalyst to be active, a certain exhaust gas temperature is required, which is often not the case during start-up and heating of the engine.

The oxidation catalyst has NO effect on the total NOx emissions, but will oxidize NO to NO2. This is useful when an oxidation catalyst is used with the particulate filter (under "combination of these systems"). It is often used in conjunction with EGR technology to reduce hydrocarbon emissions.

Method for reducing particulate emissions

The particulates form in the combustion chamber of the engine and then grow somewhat in the exhaust pipe by the smaller particles agglomerating to form larger particles and condensation of volatile materials. Very small particles are typically composed of carbon, unburned fuel, lubricating oil, metal particles, and sulfur compounds. They are carcinogenic and, due to their small volume, remain in the lungs during breathing and penetrate from the lungs into the blood. They can be transported over long distances. Thus, the requirements for particulate emissions have been reduced to very low values. In order to reduce the particulate emissions of diesel engines, the engines are provided with particulate filters. The particulate filter is installed in the exhaust system and physically traps particulates before the exhaust gas exits the exhaust pipe.

Eventually, the filter becomes filled with particulates, and therefore, as the fuel consumption increases, the flow resistance becomes higher and higher. The collected particles then need to be removed from the filter during a so-called regeneration. There are basically two ways to do this.

The particles are combusted and oxidized by increasing the temperature in a controlled manner so that the carbon in the particles is ignited and burned.

The second method is based on continuous regeneration. Such a system is called continuous regeneration

These filter systems consist of an oxidation catalyst located in front of the particulate filter. The function of the catalyst is to oxidize NO to NO2. The resulting NO2 catalyzes the oxidation of carbon to CO2 and N2. The catalyst also oxidizes HC and CO emissions and is therefore a system that reduces all emissions.

The disadvantage of this system is that over time an equilibrium must be reached between the NO2 and the particulate flow and the exhaust temperature must be above about 250 ℃ in order for the catalyst to be active. If these conditions are not met over time, the particulate filter may become saturated with particulates, increasing fuel consumption and, in the worst case, may be destroyed. Over time, the filter may become filled with ash products and then the filter may need to be replaced or cleaned. For a common truck (40 ton long haul truck) it may be required (depending on the application etc.) after about 30 ten thousand kilometres.

In many applications, it may be difficult to reach temperature requirements, such as in delivery trucks and garbage trucks that are frequently parked, low speed, and heavily idling. Then, an active system to increase the exhaust temperature is required. One common system is to inject fuel before the catalyst, then catalytically burn it and raise the exhaust temperature. However, there may still be a fundamental problem in that the exhaust gas temperature is too low for the catalyst to be active. In these cases, a burner or an electrical heating device of the system may be added.

To meet emissions regulations, large manufacturers have chosen to use EGR technology, SCR technology, or a combination of these technologies. Therefore, before making a new investment, you are advised to know the advantages and disadvantages of this technique and analyze what they may have in your own business.

Both SCR and EGR technologies have their advantages and disadvantages.

Diesel consumption can be reduced to the same level as urea injection (about 5% V in europe) using SCR technology. Assuming that the price of urea is much lower than that of diesel, the total fuel cost of the vehicle is reduced.

One way to reduce urea costs is to have its own warehouse to purchase large amounts of urea at a lower price.

Because of the recirculating exhaust gases, EGR engines may require more frequent replacement of engine lubricating oil than SCR engines.

EGR technology is known and proven. It reduces emissions from the source, i.e., the engine. The technology is also continually improved.

SCR is a positive aftertreatment system that requires additional supervision and maintenance. As the owner, you need another product to handle the operation of the vehicle and another set of systems to maintain. The weight of the vehicle increases due to the additives, which results in a lower payload of the vehicle.

SCR catalysts require a minimum operating temperature of about 300 ℃ to operate effectively. For example, this may be difficult to achieve for a vehicle with many start stops in urban traffic. When the SCR catalyst is not running, NOx emissions will be equivalent to those of a european I or european II engine.

Since particulate filters collect ash from fuels and oils, it is advantageous to use fuels with low ash content to increase the life of the filter.

SCR does not work properly and cannot be used (EGR) when the engine is heating, which means practically no reduction in NOx emissions from start-up until the engine reaches a certain temperature. The development of SCR catalysts aims at achieving operation of the catalyst at lower temperatures and it is possible to use EGR directly (almost) from the beginning. Up to now, these objectives have not been achieved. In order to obtain the most environmentally friendly solution, the selection of the emission abatement system should be made taking into account the operating conditions, but generally lack a basis for consideration. "

Thus, as described above, SCR refers to placing an SCR catalyst behind the engine. A tank containing urea solution is mounted on the vehicle. The solution was sprayed from the tank into the exhaust line before the catalyst. After proper mixing, nitrogen oxides in the exhaust gas are converted to nitrogen and water. The urea injection is electronically controlled and varies according to engine load and speed. Catalytic reduction of NOx means that the catalyst and the reducing agent added before the catalyst are converted into nitrogen (N2) and oxygen (O2) (water: H2O). The most common reductant is ammonia (NH 3), typically in the form of urea. Urea

The SCR system operates only when the exhaust gas temperature is sufficiently high. If the exhaust temperature falls below about 200 deg.c, the SCR system will no longer function and NOx reduction will be stagnant. The reduction is about 90-95% at about 300 ℃.

It is mentioned that during the start-up and heating phases, the HC emissions of the engine may be high. During normal operation, these emissions are typically very low.

The use of an oxidation catalyst readily reduces CO and HC emissions. A disadvantage is that in order for the catalyst to be active, a certain exhaust gas temperature is required, which is often not the case during start-up and heating of the engine.

It has also been previously disclosed to form particles in the combustion chamber of an engine and then subject them to some growth in the exhaust pipe by the smaller particles agglomerating to form larger particles and condensation of volatile matter. Very small particles are typically composed of carbon, unburned fuel, lubricating oil, metal particles, and sulfur compounds. They are carcinogenic and, due to their small volume, remain in the lungs during breathing and penetrate from the lungs into the blood. They can be transported over long distances. Thus, the requirements for particulate emissions have been reduced to very low values.

Particulate emissions from diesel engines are treated by particulate filters in the exhaust system, which means that particulates are trapped before the exhaust gas leaves the exhaust pipe.

Eventually, as fuel consumption increases, the flow resistance becomes higher and higher. The particles then need to be removed from the filter during a so-called regeneration. One method is to burn and oxidize the particles by increasing the temperature in a controlled manner, thereby igniting and burning the carbon in the particles.

Thus, emission control is related to reduction NOx, CO, HC and particulate emissions. The exhaust temperature determines whether the reduction was successful.

A problem inherent to today's diesel engines is that their air flow is large compared to the fuel flow, which means that the exhaust gas temperature becomes too low for the emission control to operate satisfactorily.

Today, diesel engines for vehicles generally operate according to the four-stroke principle, wherein during the intake stroke combustion air is introduced at atmospheric pressure during a higher pressure turbo charging, without being controlled for example by a throttle valve, which means that the pressure at the end of the intake stroke is at least atmospheric pressure before the compression stroke. At the end of the compression stroke, the amount of fuel required for the desired load is injected and combusted, so-called qualitative combustion, and the combustion gases expand in the working stroke during the operation of the piston. During or at the end of the working stroke, the pressure in the combustion gases is never below atmospheric pressure, which is typically 1 bar. The lower the engine load, the lower the temperature of the exhaust gas.

The following example is based on an atmospheric pressure of 1 bar at an air temperature of 0 ℃ (273K) and an effective compression ratio of 16,67, and furthermore, it is of no significance for the concept of the invention that combustion takes place at the top dead centre of the piston with a constant volume, i.e. in the otto cycle where today's diesel engines are approaching, and that this is of great importance when VVT and VCR are used in diesel engines and with the possibility of so-called quantitative combustion. Therefore, the numbers of temperature and pressure, etc. are theoretical and are not affected by heat loss, friction, etc., and end satisfactorily, but as described above, have no meaning for the concept of the present invention.

A prerequisite for the following example is that today's diesel engines are fueled at full load, increasing the compressed air mass by 2000 degrees.

Thus, there is 25% load in today's diesel engines consistent with the formula for constant volume combustion, resulting in a 0.25 x 2000 = 500 degree increase in temperature of the compressed air mass. The compression pressure becomes 51.4 bar at a temperature of 841.2K, 81.9 bar at a temperature of 1341.2K, and the pressure in the exhaust gas becomes 1.6 bar at an exhaust temperature 435K (i.e., 162 c) at the end of the working stroke. At this temperature, the SCR catalyst is no longer in an active state (temperature below 200 ℃) according to the swedish transportation authorities described above, for example. In the case of a proportionally indicated low engine load, the exhaust gas temperature naturally becomes lower. This example determines the cause of the emission control problem.

By significantly increasing the exhaust gas temperature of a diesel engine-equipped vehicle during part-load, such as during low-speed driving in urban traffic or in traffic with multiple starts and stops, or during cold start, efficient emission control can be achieved.

VCR and VVT (herein referred to as freely controllable valves), variable compression ratio and variable valve timing are described, respectively, in swedish patent SE1500404-7, which is incorporated herein by reference. The volume flow of the exhaust gas at part load can be significantly reduced while the exhaust gas temperature is still high. Thus, the exhaust gas temperature required for the normal operation of the catalyst is achieved.

Disclosure of Invention

The main object of the present invention is to provide a further improved technique which solves the problem of insufficient emission control at lower loads. This object is achieved by providing a method and an apparatus having the features indicated in the patent claims.

The present invention relates to the development of combustion technology, the earlier development not being possible.

The present invention is a further development of the solution described in SE 1500404-7.

According to a first aspect of the present invention, a method for providing high exhaust gas temperatures and/or reduced emissions at partial engine load is provided. The engine includes at least one cylinder. The cylinder has a reciprocating piston, a variable compression Volume (VCR), at least one exhaust valve, and at least one intake valve. The intake valve is provided with a Variable Valve Timing (VVT). The engine control system of a diesel engine may be configured to determine when the intake valve is open and closed based on current demands on engine power, and to adjust the compression volume to what size, so that the exhaust gas temperature at the time of exhaust is high enough to implement the cleaning function of the exhaust aftertreatment of the present invention. The present invention is characterized in that the engine control system controls the functions of the VVT and VCR such that the cylinder pressure during the working stroke reaches or falls below the current atmospheric pressure at an engine load equal to or less than 25% of the maximum engine load before or when the piston reaches the bottom dead center, wherein the intake valve is opened to introduce air to be mixed with the combustion gas when the cylinder pressure reaches or falls below the current atmospheric pressure. This aids in the oxidation of particulates, CO, HC, and increases engine operation due to increased pressure in the cylinder. To some extent, the temperature of the combustion gas increases due to the increase in pressure.

It should be understood that providing a high exhaust gas temperature refers to providing a higher exhaust gas temperature, i.e. an increased exhaust gas temperature, than conventional diesel engines. The above cylinder pressures reaching or below the current atmospheric pressure may be achieved, for example, by controlling the VVT and VCR such that when the amount of combustion air required by the engine control system has been supplied, which has been determined to be the desired engine load (according to the so-called early miller cycle), the inlet valve is closed, while the compression ratio is adjusted by the engine control system for optimum efficiency. The vent valve need not be variable. In an embodiment, the above arrangement in which the engine control system provides a sufficiently high exhaust gas temperature at the time of exhaust for the cleaning function contemplated by the exhaust aftertreatment technique of the invention may also be achieved by said early closing of the inlet valve and adjusting the compression ratio for optimum efficiency. Alternatively, a late closing of the inlet valve (after top dead center according to a so-called later miller cycle) may be used for the same purpose.

In addition to the advantages of air addition described above, the opportunity for more efficient emission control during the primary exhaust of exhaust gas also arises because negative pressure in the cylinder may cause hot exhaust gas to flow back into the cylinder, which also assists in the oxidation of particulates, CO and HC prior to re-exhaust of the exhaust gas.

The 25% load basically means that the introduction of combustion air is interrupted when 25% of the intake stroke is completed, and the effective compression ratio 16.67 is started when 25% of the compression stroke remains. As described above, the compression pressure becomes 51.4 bar at temperature 841.2K, but the combustion pressure increases to 173.5 bar at temperature 2841.2K, and the pressure in the exhaust gas at the end of the working stroke becomes 0.5 bar at exhaust temperature 530K, i.e. 257 ℃, at which temperature the SCR catalyst is still in an active state. However, most interesting is that when the pressure exceeds atmospheric pressure (here 1 bar), the temperature will become somewhat higher during the working stroke, since the exhaust gases will mainly start at this temperature. At 1 bar, the temperature is 654K, i.e., 381 ℃, at which the SCR catalyst reduces NOx by about 95%.

At 1 bar during the working stroke, 40% of the working stroke remains, and the lower the engine load, the greater part of the working stroke remains. For example, at 10% load, the temperature still becomes 654K and 75% of the working stroke is maintained through 1 bar.

It is thus possible to create a pressure in the combustion at part load of the engine that is lower than atmospheric pressure, allowing emission control measures to be taken already before the exhaust gases leave the cylinder.

The exhaust gas mass flow of the exhaust gas is low compared to the flow in today's diesel engines, the residence time in the catalyst is longer, further improving the function of the catalyst, which helps to reduce the NOx formed to nitrogen and oxygen to some extent. Furthermore, after rapid heating of the cylinder and exhaust system, hot exhaust gases can be produced directly at cold start-up, which is a substantial advantage, since the catalytic action then starts almost immediately after start-up.

In an embodiment, the incoming air is passed through a heat exchanger (Interhater ^tm ) And (5) heating. The heat exchanger exchanges heat with the exhaust gas, for example. This improves the oxidation and helps to increase the temperature in the resulting mixture. The temperature increase also results in an increase in the exhaust gas temperature compared to the case without the heat exchanger.

In an embodiment, the introduction of air is associated with the start of exhaust, wherein high velocity air flows into the cylinder and is effectively mixed with combustion gases. This may be achieved, for example, by an intake valve with Variable Valve Timing (VVT). The intake valve opens simultaneously with the exhaust valve (not necessarily variably).

According to a second aspect of the present invention, there is provided an apparatus for providing high exhaust gas temperatures and/or reduced emissions at engine part load of a diesel engine. The diesel engine includes at least one cylinder. The cylinder has a reciprocating piston allowing a variable compression volume VCR and at least one exhaust valve and at least one intake valve. The intake valve has a variable valve timing VVT. The engine control system of a diesel engine may be configured to determine when the intake valve is open and closed based on current demands on engine power, and to adjust the compression volume to what size, so that the exhaust gas temperature at the time of exhaust is sufficiently high for the intended cleaning function of the exhaust aftertreatment of the present invention. The engine control system may be configured to perform the method according to the first aspect of the invention. The device is characterized in that the pressure in the exhaust gas during the working stroke is controlled by the engine control system using the functions of VVT and VCR such that the pressure at an engine load equal to or less than 25% of the maximum engine reaches or is lower than the current atmospheric pressure before the piston reaches the bottom dead center. At this time, the intake valve is opened and air is introduced.

According to a third aspect of the present invention, a diesel engine is provided. The diesel engine includes at least one cylinder and an engine control system. The cylinder has a reciprocating piston, a variable compression volume VCR, at least one exhaust valve and at least one intake valve. The intake valve has a variable valve timing VVT. The engine control system is configured to control cylinder pressure at an engine load equal to or less than 25% of a maximum engine load using VVT and VCR functions such that the cylinder pressure reaches or falls below a current atmospheric pressure before the piston reaches bottom dead center, and to control the intake valve to open when the cylinder pressure reaches or falls below the current atmospheric pressure, thereby introducing air. The engine control system of the diesel engine may be further configured to determine when the intake valve is open and closed based on current demands on engine power, and to adjust the compression volume to what size, such that the exhaust gas temperature at the time of exhaust is sufficiently high for the intended cleaning function of the exhaust aftertreatment of the present invention.

The above-described embodiments of the method may also be used as respective embodiments of the second and third aspects of the invention.

Drawings

The foregoing and other aspects of the invention will now be described in more detail, with reference to the appended drawings showing embodiments of the invention, in which

FIG. 1 shows a flow chart illustrating an embodiment of a method according to the first aspect of the invention, an

Fig. 2 schematically shows an embodiment of the device according to the second aspect of the invention.

Detailed Description

Fig. 1 shows a flow chart schematically illustrating an embodiment of a method according to the first aspect of the invention, wherein the method comprises determining (1) when an inlet valve is to be opened and closed based on current demands on engine power, and adjusting the compression volume to what size such that the exhaust gas temperature at the exhaust is high enough to achieve the desired cleaning function of the exhaust aftertreatment of the invention. The method further comprises controlling (2) the functions of VVT and VCR using the engine control system such that when the engine load is equal to or less than 25% of said maximum engine load, the cylinder pressure during the working stroke reaches or is below the current atmospheric pressure before or when the piston reaches bottom dead center, wherein when said cylinder pressure reaches or is below said current atmospheric pressure the inlet valve is opened (4) to introduce air to mix with the combustion gases. Before the air is introduced, the air is heated (3) by heat exchange with the exhaust gases.

Fig. 2 shows an embodiment of the device according to the second aspect of the invention, but can also be considered as showing parts of an embodiment of a diesel engine according to the third aspect of the invention.

The engine is provided with cylinders 11 in a conventional manner. A piston 12 connected to a piston rod 13 moves back and forth in the cylinder 11. The engine further comprises a variable compression volume 14 formed in a cylinder head in a slave cylinder 15. The variable compression volume 14 opens downwardly towards the cylinder 11 and is provided with a secondary piston 16 which allows for the reciprocating movement of the variable compression Volume (VCR). By varying the position of secondary piston 16, the total volume above piston 12 is varied. The secondary piston is adjustable by means of an actuator 17. At least one exhaust valve 18 and at least one intake valve 19 are arranged in the cylinder head. At least the actuator 20 is used to provide variable valve timing VVT for the intake valve. In the figure, the exhaust valve 18 is shown with an actuator in a corresponding manner to the intake valve, but this is not required. It is also possible to drive the exhaust valve conventionally using a camshaft. Different types of actuators suitable for use as

actuators

17, 20 are known and will not be described in detail here. The injector 23 is arranged to inject fuel into the variable compression volume 14.

The engine and apparatus also include an engine control system 21. The engine control system 21 determines when the intake valve 19 is open and closed based on current demands on engine power and to what size the compression volume 14 is adjusted so that the exhaust gas temperature at the time of exhaust (i.e., when the exhaust valve 18 is open) is high enough to provide and maintain the desired cleaning function of exhaust aftertreatment (e.g., SCR). The engine control system 21 is configured to control the cylinder pressure to reach or fall below the current atmospheric pressure at an engine load equal to or less than 25% of the maximum engine load before the piston 12 reaches the bottom dead center using the functions of VVT and VCR (i.e., controlling the opening and closing times of the intake valves and the position of the secondary piston 16 by using the actuators 17, 20). The engine control system 21 is also configured to control the opening of the intake valve 19 using the actuator 20 when the cylinder pressure reaches or falls below the current atmospheric pressure, thereby introducing air.

In the figure, it is shown when the piston 12 is just above bottom dead center, i.e. at the end of the working stroke. The cylinder pressure is now lower than the current atmospheric pressure, controlled by the engine control system 21. Both the inlet valve 18 and the outlet valve 19 are open, so that air is introduced via the inlet valve and hot exhaust gases are introduced via the outlet valve (see arrows in the figure). The air introduced through the intake valve is heated by exchanging heat with the exhaust gas using a heat exchanger 22 (shown schematically).

When the piston 12 eventually begins to move upward (after bottom dead center), the intake valve 19 closes, while the exhaust valve 18 remains open to exhaust combustion gases (during the exhaust stroke).

The invention is not limited to the embodiments described above but may be modified within the scope of the appended claims. For example, variable compression volume and variable valve timing may be achieved in many different ways and with many different types of actuators (pneumatic, hydraulic, electric). The engine control system need not be configured entirely as described above. For example, the exhaust valve and the intake valve need not be opened simultaneously, but the intake valve may be opened and closed before the exhaust valve is opened.

Claims

1. A method for providing increased exhaust gas temperature and reduced emissions in case of an engine part load in a four-stroke diesel engine reaching 25% or less of a maximum engine load, the engine comprising at least one cylinder with a reciprocating piston, a variable compression volume and at least one exhaust valve and at least one intake valve with a variable valve timing, wherein an engine control system determines (1) when the intake valve is opened and closed and to what size the variable compression volume is adjusted according to current demands for engine power such that the exhaust gas temperature at the exhaust gas is sufficiently high to achieve a desired cleaning function of a current exhaust gas aftertreatment, characterized in that the engine control system uses the functions of the variable valve timing and the variable compression volume to control (2) the cylinder pressure during the working stroke to reach or drop below a current atmospheric pressure before the piston reaches a bottom dead center when the cylinder pressure reaches or drops below the current atmospheric pressure, and to introduce the combustion gas (4) with the intake valve air pressure at the time of the current exhaust gas aftertreatment.

2. A method according to claim 1, characterized in that the introduced air is heated (3) by the exhaust gases in a heat exchanger.

3. A method according to claim 1, characterized in that the inlet valve is opened (4) for introducing air at the beginning of the exhaust.

4. An arrangement for providing an increased exhaust gas temperature and a reduced emission at an engine part load in a four-stroke diesel engine, the engine comprising at least one cylinder (11) with a reciprocating piston (12) allowing a variable compression volume (14), at least one exhaust valve (18) and at least one inlet valve (19), the inlet valve being provided with a variable valve timing, wherein the arrangement comprises an engine control system (21) which determines when to open and close the inlet valve (19) and to what size the variable compression volume (14) is adjusted based on current demands on engine power, such that the exhaust gas temperature at the time of exhaust is sufficiently high to achieve a desired cleaning function at the time of current exhaust gas aftertreatment, and for performing the method according to claim 1, characterized in that the engine control system (21) is configured to control the cylinder pressure to reach or drop to the current air pressure before the piston reaches or drops to the current air pressure at or below the current air pressure at the time of the cylinder load reaches 25% or less of the maximum engine load.

5. The device according to claim 4, further comprising a heat exchanger (22), the heat exchanger (22) being arranged to raise the temperature of the air introduced via the inlet valve (19) when the cylinder pressure reaches or falls below the current atmospheric pressure.

6. The apparatus of claim 4 or 5, wherein the engine control system (21) is configured to control the intake valve (19) such that the intake valve opens when cylinder pressure reaches or falls below the current atmospheric pressure and begins to exhaust.

7. Diesel engine comprising at least one cylinder (11) with a reciprocating piston (12), a variable compression volume (14) and at least one exhaust valve (18) and at least one inlet valve (19) provided with a variable valve timing, and a device according to any of claims 4-6.